2 The Microscope. 



ate combination of circumstances which makes a certain objec- 

 tive better than its fellows, but this is a rare exception, and is 

 positive evidence that the exact requirements of the formula 

 have been complied with. As a rule, therefore. I believe that the 

 opticians' claims may be relied upon, and where the results in 

 the hands of the microscopist do not correspond with them, the 

 cause may usually be looked for in the lack of experience in 

 manipulation or in conditions, which differ from those under 

 which the objective was completed. The belief, which I am 

 aware is extant, that there are great differences in objectives 

 purporting to be similar, is, in my opinion, not justified, at any 

 rate in the productions of those men who, by general acknowl- 

 edgement, are at the head of their profession. I admit that, as 

 in everything which depends upon human skill, there is strictly 

 speaking, no absolute uniformity, but also claim, that with few 

 exceptions, the differences are so slight, that anything but the 

 most expert manipulation cannot detect them. 



It therefore appears to the writer that any information 

 which will tend to improve the knowledge of testing objectives 

 will not only prove beneficial to the microscopist, but will 

 prove advantageous to the optician, in that his work will receive 

 a fair trial, based upon a knowledge of the principles involved, 

 and that he may be convinced that all his work which deserves 

 commendation will be the better appreciated. The following 

 points are by no means new, but are often lost sight of in 

 making tests. The writer will speak of medium and high 

 power objectives only, as the deleterious influences are most 

 noticeable in these, but they apply as well to the lower powers 

 though in a less degree. 



The part of the instrument which has a strong bearing in the 

 performance of the objective is the mirror. It should be ad- 

 justable on the mirror bar, so that it can be accommodated to 

 the variations in distance of the source of light from the instru- 

 ment. When parallel rays are used, as with light from the 

 sun or clouds, its distance from the object should be decreased 

 and increased when lamplight is used. It should be exacted 

 that the focus of the concave mirror be within the limits of its 

 adjustment. The serious disadvantage of its incorrectness in 

 this respect can easily be seen by taking, for instance, a 1-5 



